1. Field of the Invention
The present invention relates to an electromagnetic-wave suppressing radiator sheet efficiently transmitting heat from a heating element such as a LSI package to radiator parts such as a radiator plate, a heat pipe, or a heat sink and suppressing electromagnetic waves from coupling with each other. The present invention also relates to an electronic apparatus using such an electromagnetic-wave suppressing radiator sheet.
2. Description of the Related Art
In recent years, electronic apparatuses have been small-sized. Since electric energy (the amount of heat generated) may not be changed much due to the diversity of application, it is important to take measures for heat radiation in the apparatuses.
Radiator plates, heat pipes, heat sinks, and so on made of metal materials with high heat conductivities, such as cupper and aluminum, have been widely used as measures for heat radiation (or measures against heat) in electronic apparatuses.
The arrangement of such a radiator part with excellent heat conductivity for a heat-producing part (high-temperature place) in the electronic apparatus or the arrangement thereof for the area extending from the heat-generating part (high-temperature place) to a low-temperature place can lead to improve a radiation efficiency or temperature relaxation in the apparatus.
However, the radiator part with excellent heat conductivity is made of a metal material, so that it will work as an antenna for high-harmonic noise components or as a communication channel for high-harmonic noise components.
Therefore, the radiator part may also gather the high-harmonic component of an electric signal as a disadvantageous effect. As a result, it often causes the radiation of unnecessary electromagnetic waves.
In addition, a heat-producing part (high-temperature place) in the electronic apparatus mainly includes a chip (semiconductor package) or the like with high current density. In other words, the higher the current density becomes, the higher the electric intensity or magnetic intensity which may serve as a component of the radiation of unnecessary electromagnetic waves increases.
On the other hand, it may be difficult to contact the radiator part with a large surface area of the chip due to their respective dimensions and surface conditions. In such a case, the efficiency of heat radiation may be reduced due to a decrease in contact area or generation of a space between the radiator part and the chip.
Accordingly, the space between the chip (semiconductor package) and the metal radiator such as a heat sink may be filled with a high-temperature conductive filler (heat conductive sheet).
For example, the high-temperature conductive filler with excellent thermal conductive and filling properties can be prepared by allowing a polymer material to contain a material with high heat conductivity, such as alumina or aluminum nitride as a filer.
However, even the use of such a high-temperature conductive filler may not suppress the radiator part picking up the high-harmonic components of electric signals.
As schematically illustrated in FIG. 1, if a high-temperature conductive filler (heat-radiating filler sheet) 53 is placed between a chip 51 and a radiator part (radiator plate) 52, a large amount of heat conduction 62 occurs from the chip 51 to the radiator plate 52 through the heat-radiating filler sheet 53.
In addition, a magnetic field 61 caused by the chip 51 is coupled with the radiator plate 52 to allow a signal 63 with frequency components to be conducted in the radiator plate 52, causing the radiation of unnecessary electromagnetic waves 64.
In order to prevent the coupling of the magnetic field, a space filler (electromagnetic-wave suppressing radiator sheet) prepared by mixing the space filler with a magnetic material may be used. The electromagnetic-wave suppressing radiator sheet may contain a material with high heat conductivity, such as alumina or aluminum nitride, as a filler and a high-permeability material such as ferrite in a silicon- or acryl-based polymer. Accordingly, the electromagnetic-wave suppressing radiator sheet can be provided with a combination of excellent high heat conductivity and an effect of suppressing electromagnetic waves (effect of decoupling a magnetic field).
As schematically illustrated in FIG. 2, when an electromagnetic-wave suppressing radiator sheet 54 is placed between the chip 51 and the radiator part (radiator plate) 52, the electromagnetic-wave suppressing radiator sheet 54 can suppress the coupling of the magnetic field 61 caused by the chip 51 with the radiator plate 52. It leads to a decrease in the signals 63 with frequency components generated in the radiator plate 52, thereby reducing the radiation of unnecessary electromagnetic waves 64.
However, the contents of material powder with excellent heat conductivity may be reduced by combining a radiator sheet with a magnetic material, resulting in a decrease in heat conductivity of an electromagnetic-wave suppressing radiator sheet.
Therefore, the arrangement shown in FIG. 2 leads to a decrease in the heat conduction 62 from the chip 51 to the radiator plate 52 through the electromagnetic-wave suppressing radiator sheet 54 and also causes a decrease in the heat conduction 62 in the radiator plate 52.
Flexibility may also be required for the heat-radiating filler sheet and the electromagnetic-wave suppressing radiator sheet to implement these sheets easily and reduce the thermal resistance caused by the contact between such sheets and the heating element and the radiator part. Therefore, the contents of radiator powder or magnetic powder in the sheet are limited.
In order to suppress a decrease in heat conductivity as small as possible, magnetic particles with excellent magnetic properties may be used to reduce the volume of the magnetic material in the sheet to the absolute minimum.
Ferrite powder, which is metal oxide, is mainly used as magnetic powder in the electromagnetic-wave suppressing radiator sheet to obtain insulation properties.
Metal magnetic materials have excellent magnetic properties. However, metal powder may be exposed on the surface of the sheet when mixing such magnetic material in the sheet. The use of such a material in the electronic apparatus may cause an electric short circuit.
Ferrite powder mixed in the electromagnetic-wave suppressing radiator sheet may be in a size of micrometers.
The micrometer-size ferrite powder is mainly prepared by pulverizing the bulk of sintered ferrite obtained by the solid reaction method. Therefore, the resulting ferrite powder will deteriorate in magnetic properties, compared with the bulk of sintered ferrite, because of crystal distortion in the ferrite particles during the pulverizing.
As described above, the bulk of sintered ferrite has the magnetic properties higher than that of ferrite powder with a size of micrometers. Thus, a stacked structure in which a ferrite plate is placed in a radiator sheet has been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2001-15656). With such a structure, the ferrite plate obtains a comparatively high effect of suppressing electromagnetic waves while securing somewhat high conductivity by lowering the volume ratio of the magnetic material.